The present invention relates to register indicia and to control systems for adjusting the extent to which printed images overlap. More particularly, the invention relates to method and apparatus for detecting misregister and automatically registering two or more printed images.
In multi-colored printing, color images are produced by overprinting several images, each printed in different colors. To provide the proper effect, the several differently colored images should be aligned or "registered" quite precisely atop one another. To control this, the various printing units which together make up the multi-color press include adjustment mechanisms enabling one image to be moved relative to another. In order to set these adjustments properly, some technique must first be provided for detecting misregistration between the differently colored images.
The simplest method of detecting misregistration is for the pressman to visually study the printed product to identify the nature and extent of any misregistration between the images. This manual misregistration detection and adjustment technique allows great flexibility and permits the pressman to interject his own experience into the registration process. Manual registration adjustment is therefore widely practiced, either alone or in conjunction with automated systems.
Automated systems have some advantages over manual misregistration methods, principally in the speed with which they operate. Upon the initial start-up of a multi-color press some misregistration generally exists between the various printed color images. All of the printed product produced by the press until this misregistration is corrected is discarded as waste. It is therefore desirable to eliminate misregistration as rapidly as possible in order to reduce the extent of paper waste. Other factors requiring adjustment, notably color density, also contribute to paper waste.
Because of this, a variety of automated systems have been provided for detecting and correcting misregistration. Uniformly, these system require indicia separate and apart from the printed image, per se, in order to simplify the automated process of detecting misregistration. Most generally these indicia take the form of individual lines printed by the various units of the press concurrently with the images. The positional relationship between the register indicia lines is directly indicative of the registration between the corresponding printed images. Due to inconsistencies in the printing process, however, the register indicia lines have varying width and density, rendering accurate and repeatable automated determination of their position difficult.
Automatic measurement of the positional relationship between the two register indicia was inherently a dynamic process. One or more sensor was mounted on the press to detect the passage of the register indicia, and the time between passage of the first and second register indiciums was equated with physical displacement between the two indicia. The measurement was therefore press-speed dependent.
Independently of registration control, color bars have been used in the past for ink density control and press operation diagnostics. The color bars have been printed on the web concurrently with the printing of the image, usually in the margins between the images on the web. No unified system has been used, however, for dealing with both color and registration control; the two have historically been treated as separate problems with separate printed indicators and separate control processes.